Acid-catalyzed photoresist compositions are widely used in photolithography applications, especially in the manufacture of integrated circuits and other devices where patterned photoresist layers are employed in device manufacture.
In most applications, the acid-catalyzed photoresist composition is applied to a surface where a patterned photoresist layer is desired (e.g. to the surface of a semiconductor wafer) as a liquid solution. The solvent is then typically removed to form a thin solid photoresist layer on the desired surface. In many instances, the solvent removal may be assisted by a heating step. Depending on the specific photoresist composition, heating may also be used to cause a reaction in the unexposed photoresist to improve its properties (e.g. by crosslinking). Typically, the applied photoresist layer is then patternwise exposed to radiation adapted to cause generation of acid or other property change within the exposed areas of the photoresist. The exposure is typically done through a mask containing a pattern of openings to create a corresponding pattern of exposed areas in the photoresist. In some cases, such as with the use of electron beam (e-beam) radiation, a patternwise exposure may be achieved without a mask by scanning the electron-beam over the photoresist layer in a patterned manner.
After radiation exposure, the pattern is developed or revealed by selective removal of the exposed or unexposed portions of the photoresist depending on whether the photoresist acts in a positive mode or a negative mode. Prior to selective removal, the exposed photoresist layer may be treated (e.g., by application of heat) to enhance the property differences created by the exposure. Once the exposed photoresist layer is ready for development, the selective removal is typically done by treating the photoresist layer with a solvent which selectively removes portions of the photoresist by dissolution. The patterned photoresist layer may then be used in whatever manner desired for the specific manufacturing objective.
Acid-catalyzed photoresists are generally characterized by the combination of an acid-sensitive polymer with a radiation-sensitive acid-generating compound (photosensitive acid generator or PAG). On exposure of the photoresist composition to a suitable radiation source, the PAG generates an acid of sufficient strength to cause a reaction with the acid-sensitive polymer. The reaction is typically catalytic in nature (i.e. the initial acid generated reacts with the acid-sensitive polymer to create additional acid which is available for further reaction). The acid-catalyzed reaction is often enhanced by baking the exposed photoresist composition. The reaction between the acid-sensitive polymer and the generated acid alters the characteristics of the polymer in the exposed photoresist composition relative to the same polymer in an unexposed photoresist composition such that the exposed photoresist can be selectively removed relative to the unexposed photoresist (in the case of positive photoresists).
In the case of positive photoresist compositions, the acid generated on exposure to radiation typically causes the exposed photoresist composition to exhibit increased solubility in alkaline media (and/or other property difference) compared to the unexposed photoresist. The acid-sensitive polymer typically contains acid-labile pendant groups and polar pendant groups. The polar pendant groups promote solubility of the polymer in alkaline media (e.g. an aqueous base solution) whereas some or all of the acid-labile pendant groups "protect" the polymer from solubility in alkaline media. In the unexposed polymer, the relative amounts of the polar pendant groups and the acid-labile protecting groups are preferably such that the unexposed polymer remains substantially insoluble in alkaline media. On exposure of the photoresist to suitable radiation, at least a portion of the acid-labile groups are cleaved from the polymer in response to the generated acid. This cleaving reaction shifts the balance between the polar groups and the acid-labile protecting groups such that the polymer becomes substantially soluble in alkaline media.
The nature of the acid-labile groups is important to the degree and ease of the solubility shift on radiation-induced acid generation. The ability of the polymer to undergo the shift in solubility is enhanced if the cleaving of the acid-labile groups results in formation of additional pendant polar groups on the polymer. The performance of the photoresist is also enhanced to the extent that the cleavage of the acid-labile group results in the formation of additional acid which causes cleavage of further acid-labile groups from the polymer.
The nature of the pendant polar groups on the original polymer also has a substantial impact on the performance of the photoresist. The polar groups should be of sufficient strength to impart substantial alkaline solubility when present in a sufficient proportion while they should not adversely impact the other performance aspects of the photoresist. Over the last several years, the most widely used (especially for deep UV (190-315 nm wavelength lithography) pendant polar groups have been based on imparting polar functionality to a pendant aromatic ring (e.g., of a styrene monomer) which is incorporated into the photoresist polymer. The polar functionality can be imparted on the aromatic ring of the styrene before or after incorporation of the styrene in the synthesis of the photoresist polymer. The most widely used polar-functionalized aromatic group has been hydroxystyrene, however the use of other pendant polar groups based on styrene. Polar-functionalized styrene groups also tend to provide improved performance characteristics in subsequent processing.
While the acid-catalyzed photoresist compositions available in the art provide good performance for many photolithography applications, there remains a desire for improved performance such as greater change in solubility on radiation exposure (e.g. for improved pattern contrast), reduced shrinkage of the photoresist composition on exposure and on post-exposure baking, improved kinetics of solubility change (e.g. to reduce processing time), and improved etch resistance of the patterned photoresist layer. Reduced shrinkage and improved etch resistance are especially important in the reliable production of finer sized features.
In many photolithographic applications, the desired size of feature details in the exposed pattern continues to become increasingly finer. For semiconductor applications, there is typically a desire to create patterns having resolution of details in the sub-micron range. This desire is especially strong in the manufacture of integrated circuits since the reduction of detail size generally corresponds to an increase is device density that can be constructed. Similar desires for the ability to produce fine details exist in other areas such as the manufacture of micromachines, magnetic heads, magnetic/electronic storage devices, etc.